Here he is in the Safety 1st, the red line is the top harness slot.
Sorry for the crappy cell phone picture in front of the picture window, it's hard to get him to sit in a car seat when he knows we're not going anywhere, lol.
It looks like there's at least 2 inches between the top harness slot and the top of his shoulders right now...so he should have at least 2 years in that seat. Do you think there's 2 inches..or more...above his shoulders? And when will he be 4 years old? We often say around here that you have about 1 inch of torso growth per year, so inch per year.
It's a nice fit.